Iphone 5c - Bad ESN can still use Facetime and iMessage?

So about 2 months ago i bought a week old brand new Sprint iPhone 5c for $200. It had a bad esn & i was not able to unlock it for tmobile.
I suspect its due to non-payment. since in badesncheck.com everything else comes up good.

That doesnt really bother me. I gave it to me lil niece for christmas as she wanted an iPod 5g anyway.

However, the guy who sold it to me gave it to me with the original nano sim card.
& after these 2 months ive noticed that i can still use it to make calls (provided the sim card is in the phone).
In theory i think i could keep using the phone as my own until service on it is cut off or even run up the bill by making international calls since the account belongs to the guy who sold it to me.

I also recently noticed that the phone number from the phone is connected to iMessages and Facetime.
& so i can send messages back and forth and have people call me to the number even when i dont have the sim card in, as long as im on wifi.

Tho im not complaining, I feel a little confused and was wondering if anybody could maybe clarify why i can still use this phone number to make calls, or have people message me at that phone number thru imessages and facetime, if the phone has a bad esn for non-payment.

I thought if it had been listed as bad esn by sprint they would cut off the cellular service. i also figured they would block any type of messages and facetime associated with that very phone number that is the one that has a bad esn.

Also i was wondering if, 1) we can just keep using that phone number thru wifi services like the ones mentioned until they cut it off without any real repercussions, since the account belongs to the guy who initially sold it to me; 2) if so, about how long do u think this will last before we can no longer use his cellular service to make calls, and before people can no longer send messages or facetime to that phone number; 3) once the phone has no more service or wutever it is they will do once its been an extended period of time without it being paid off, will people still be able to reach us through iMessages and Facetime at the number that is connected to those services (through wifi)?

At the moment it simply feels as if its a free phone with free service until its cut off, then it really will just be an ipod touch. lol

I can only assume that the reason the device's phone number still works with iMessage, and FaceTime is because Apple has it tied to those services. I doubt you are able to use the number for cell calls, but I could be wrong. Personally, I wouldn't want to use someone else's number and would attempt to restore the device as new and use my own Apple ID for iMessage and FaceTime, but that's Just me, D. As for how long the number will work? No one can say for certain.